With the rapid spread of Long Term Evolution (hereinafter referred to as LTE) and LTE-Advanced, it is becoming possible to provide full-scale mobile broadband services. In order to support rapidly increasing traffic in cellular networks, it is necessary to further promote ultra-high speed and large capacity properties of the fifth generation (5G) mobile communication system and to improve frequency utilization efficiency as compared with LTE. Highly efficient radio access technology is necessary in addition to heterogeneous networks which overlay small cells that efficiently accommodate non-uniform traffic in macrocells. It is also necessary to further promote ultra-high speed and large capacity properties of the backhaul between base stations and a Serving-Gateway (S-GW) in an Evolved Packet Core (EPC) network, in addition to the ultra-high speed and large capacity radio access network which achieves gigabit-class services to User Equipment (UE). A backhaul link is composed of an E1/T1 dedicated line, an optical fiber network, a microwave wireless backhaul, and the like. The wireless backhaul has an advantage of lowering the network cost compared with wired backhaul. The same can be said for the fronthaul which connects Remote Radio Equipment (RRE) to the centralized base station in a configuration in which the centralized base station performs processing of the base station composed of the RRE and processing of a physical layer and a higher-level layer of a baseband.
In a wireless backhaul using microwaves, frequency utilization efficiency has been improved by increasing a modulation level in a signal space arrangement and by using Multiple-Input Multiple-Output (MIMO) multiplexing using vertical polarization and horizontal polarization. The Rectangular or Cross QAM signal space arrangement (constellation) is implemented in a wireless backhaul using microwaves, because the Euclidean distance between signal points can be increased. When the number of signal points is 22k, the Rectangular constellation is used. When the number of signal points is 22k+1, the Cross constellation is used.
FIG. 1 is a diagram showing an example of a Rectangular 16 QAM constellation when k=2. In the Rectangular QAM constellation, labels of adjacent symbols, i.e., sets of information bits each representing a symbol, can achieve gray mapping, which differs by only one bit.
FIG. 2 is a diagram showing an example of a Cross 32 QAM constellation when k=2. The Cross QAM constellation is pseudo gray mapping, because full gray mapping cannot be obtained. The Rectangular or Cross QAM constellation can maximize the Euclidean distance between signal points as compared with other constellations. Therefore, these constellations are implemented in actual systems such as wireless backhauls and cellular systems.
The most significant degradation factor of characteristics in the wireless backhaul is phase noise of an RF frequency of an oscillator output of a frequency synthesizer. The Circular QAM having a concentric constellation has attracted attention in terms of its tolerance to phase noise. The Circular QAM also has an advantage of achieving a lower Peak-to-Average Power Ratio (PAPR) as compared with the Rectangular or Cross QAM. The Circular QAM is also referred to as Star QAM or Amplitude and Phase Shift Keying (APSK). The Circular QAM constellation is composed of N concentric rings. Each ring is subjected to Phase Shift Keying (PSK) processing. The Circular QAM signal space arrangement can be divided roughly into the case in which all rings have the same number of signal points and the case in which the number of signal points differs according to the ring. In the signal point arrangement in which all rings have the same number of signal points, the phases can be set commonly for all rings.
FIG. 3 is a diagram showing an example of the Circular 16 QAM constellation of 8×2 rings. As shown in FIG. 3, bits representing a symbol label can be independently mapped to bits representing amplitude or phase information. In the case of Circular 16 QAM, one of four bits of a symbol represents the amplitude information, and the other three bits represent the phase information. The Circular 16 QAM has an advantage of optimizing the ring amplitude ratio, which is a parameter that most affects Bit Error Rate (BER) characteristics, for only the bit representing the amplitude information. The Circular 16 QAM can also achieve full gray mapping, because the bits representing the amplitude and phase information can be mapped independently. However, the Circular 16 QAM has a disadvantage that the inner the ring is, the more decoding errors of the bits representing the phase information occur.
In the signal space arrangement in which the numbers of signal points on the rings differ from each other, it is possible to set the phase interval on the same ring substantially constant regardless of the ring by changing the number of signal points on the same ring. On the other hand, since each bit representing a symbol label simultaneously represents the amplitude and phase information, full gray mapping is difficult in general and instead pseudo gray mapping is used.
In the wireless backhaul using microwaves, a value higher than 0.9 is used for an overall channel coding rate. Since a coding gain is reduced due to a high channel coding rate, partial channel coding is used in which channel coding is applied to only a fixed number of bits from the least significant bit (LSB) in a symbol. In the partial channel coding, double gray mapping is used, in which gray mapping is performed independently on coded bits and uncoded bits. Since the coding rate can be reduced by channel coding a fixed number of bits from the LSB the coding gain can be increased.
FIG. 4 shows a configuration example of a transmission unit which uses the double gray mapping. When the number of information bits indicating a symbol label is m, the channel coding is performed on n bits from the LSB in ascending order. Thus, the remaining upper bits (m−n) are mapped to a symbol without being coded. When the coding rate of n bits which are subject to the channel coding is denoted by r, the overall channel coding rate R of all information bits is expressed by Equation 1.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            R        =                  1          -                                    (                              1                -                r                            )                        ⁢                          n              m                                                          (                  Equation          ⁢                                          ⁢          1                )            
By reducing the effective coding rate r, the coding gain is increased, and thus the decoding error of the lower n bits can be reduced.
In the Circular QAM signal space arrangement in which the number of signal points in each ring is equal, each bit represents either the amplitude or phase information. Thus, for example, the Log-Likelihood Ratio (LLR) of the channel-coded amplitude bits does not contribute to the improvement of the reliability of uncoded phase bits. The same is true for the reverse combination of the amplitude bits and the uncoded phase bits. Therefore, in the Circular QAM, parallel double gray mapping is used in which double gray mapping is performed independently on the amplitude bits and phase bits.
FIG. 5 is a diagram showing a configuration example of a transmission unit which performs parallel double gray mapping of the Circular QAM. In the parallel double gray mapping, na and np bits are mapped to the amplitude bits and phase bits, respectively, among the lower n bits on which the channel coding is performed. Likewise, (ma−na) and (mp−np) bits are allocated to the amplitude bits and the phase bits, respectively, among (m-n) uncoded bits. As shown in FIG. 5, bits representing an amplitude and a phase are independently subject to the gray mapping separately for the coded bits and uncoded bits.
In the partial channel coding in which double gray mapping is carried out, cooperative decoding is performed.
FIG. 6 is a diagram showing a configuration example of a reception unit which performs the cooperative decoding. The reception unit shown in FIG. 6 first decodes the coded bits which have been subjected to the channel coding. The coded bits are considered to be more reliable than the uncoded bits because of the coding gain of the channel coding. The a posteriori LLR of a decoder output of the coded bits is used to reduce the number of symbol candidates for the uncoded bits. The reception unit performs double gray coding mapping in advance so that the Euclidean distance between the surviving symbol replica candidates after the reduction is increased. Thus, when the lower coded bits can be decoded without an error, the Euclidean distance between the surviving symbol replica candidate is increased, which enables the upper bits to be decoded with high reliability.
FIG. 7 shows an example of BER characteristics with respect to received Signal-to-Noise power Ratio (SNR) in an Additive White Gaussian Noise (AWGN) channel added with phase noise of a 64×16 Circular 1024 QAM having 16 signal points in each ring. FIG. 7 also shows the characteristics of Rectangular 1024 QAM for comparison. In 1024 QAM, m=10 information bits represent 210 symbol labels. In Circular 1024 QAM, 4 bits represent the amplitude information, and 6 bits represent the phase information. The turbo coding has been used for the channel coding. The overall coding rate is R=9/10, and the actual coding rate of the channel coding bits is r=5/6. The Max-Log-MAP (Maximum a posteriori probability) decoding has been used for the decoding. The partial turbo coding has been used. The number of turbo coding bits is n=6. Specifically, the lower 2 bits of the amplitude bits and the lower 4 bits of the phase bits have been subjected to the channel coding. The phase noise has been approximated by an Autoregressive Moving Average (ARMA) model. The phase noise power level at 0 Hz is −40 dBc/Hz. As shown in FIG. 7, in the AWGN channel in which phase noise is taken into consideration, the required received SNR of the Circular 1024 QAM to satisfy BER of 10−5 is degraded by about 1 dB as compared with the Rectangular 1024 QAM. The Circular QAM has high tolerance to phase noise as compared with the Rectangular QAM. That is, in the Circular QAM, the degradation of BER when there is no phase noise is small as compared with when phase noise is taken into consideration. However, in the Circular QAM, since the Euclidean distance between signal points on the concentric inner circles is reduced, the decoding error is large. Therefore, the BER characteristics are degraded in the Circular QAM as compared with the Rectangular QAM.